In telephone circuits which have a telephone coupled by an interface transformer to transmission equipment, it has been a practice prior to this invention to employ a relay switching circuit which normally connects the telephone to its associated winding of the transformer and which maintains the telephone normally disconnected from a ringing generator in absence of a ring control signal. Upon the occurrence of the ring control signal, a relay in the relay switching circuit is effective to disconnect the telephone from its associated transformer winding and to connect the telephone to the ringing generator so that the a.c. ringing voltage, which is produced by the generator, can energize the ringer in the telephone.
The purpose of disconnecting the telephone from the generator when the telephone is connected to its associated transformer winding is to prevent current flowing in the talking circuit from being shunted or diverted through the generator which is usually of low impedance. A purpose of disconnecting the telephone from its associated transformer winding when the telephone is connected to the ringing generator is to prevent ringing current, which is fed to operate the ringer, from being diverted through the transformer winding.
Though these relay switching circuits operate generally satisfactorily, it was recognized as an incident to this invention that cost and space savings could be realized over the foregoing type of relay switching circuit.